Filaments, such as monofilaments or multifilament yarns, made of polyester are generally obtained by melt spinning a polyester and the monofilament obtained is subsequently subjected to a drawing operation, in order to orient the structure of the polyester and to obtain high mechanical properties, such as, for example, the Young's modulus or the tenacity. The drawing operation is carried out either in a single stage or in several stages. The total draw ratio applied is generally of the order of 6.
However, for applications of monofilaments as, for example, reinforcing components for straps, conveyor belts or tires or for the production of felts for paper machines or fabrics for screen printing, and the like, it is advantageous and desired to obtain still higher mechanical properties.
Current processes for the manufacture of monofilaments are limited because it is impossible to apply high draw ratios to the polyester filament without causing the latter to break; the maximum ratios are thus of the order of 7 to 8.
In fact, numerous processes for drawing polyester monofilament have been disclosed in the literature. Thus, mention may be made, by way of example, of Japanese Patent J02091212, which discloses a two-stage drawing operation, the first drawing operation being applied according to a ratio of between 3.5 and 5 and an overdrawing operation subsequently being applied. The overall draw ratio is then between 5 and 5.8.
U.S. Pat. No. 3,998,920 also discloses a two-stage drawing process with a first draw ratio of between 4 and 6 and a total draw ratio of between 6 and 7.5.
Drawing processes equivalent to those described above are also made known in U.S. Pat. Nos. 3,963,678, 4,009,511, 4,056,652, 5,082,611 and 5,223,187.
Furthermore, in the usual industrial processes, the drawing operation is generally carried out in a single stage, optionally followed by an overdrawing stage and/or by a relaxation stage.
The monofilaments obtained by these drawing processes exhibit a high level of mechanical properties, for example a breaking stress of the order of 600 MPa and an elongation at break of the order of 30%.
These filaments exhibit a stress at 4% elongation of less than 500 MPa and a small elastic range generally corresponding to an elongation of less than 4% and to a stress of less than 300 MPa.